1) Field of the Invention
This invention pertains to the field of solar cell arrays, and more particularly, to a system and method of arranging solar cells to increase output power of a solar cell array.
2) Background and Summary of the Invention
As the supplies of fossil fuels continue to be subject to volatility, alternative energy sources continue to be investigated. In particular, solar energy is seen as a large potential source of energy which remains as yet largely untapped. Accordingly, research continues into improved solar energy devices, including particularly, solar cells and solar cell arrays.
The most common form of solar cells is based on the photovoltaic (PV) effect in which light falling on a two-layer semiconductor device produces a photovoltage or potential difference between the layers. A single PV solar cell typically produces a working output voltage of about 0.5 volts and a current that changes depending upon the intensity of light to which the cell is exposed. For example, a PV solar cell might produce a current of 100-500 mA when exposed to bright light.
An important characteristic of such a solar cell is that the cell""s working output voltage does not depend on its size, and also remains fairly constant with changing light intensity on the cell. However, the current produced by a PV solar cell is almost directly proportional to its size and to the intensity of light received by the cell. Also, although the current output by the cell is relatively stable at higher temperatures, the working output voltage drops at higher temperatures, producing a drop in solar cell output power as the cell temperature increases.
Because, as noted above, a single PV solar cell has a typical working voltage of about 0.5 V, typically a plurality of such cells are connected together in series to provide larger working voltages. Currently, there are three basic categories of devices: (1) low voltage/low power devices made by connecting 3-12 small PV cells (typically amorphous silicon devices) in series to produce 1.5-6V; (2) small solar panels having many cells connected in series and producing 3-12V and 1-10 watts; and (3) large solar panels, usually composed of 10-36 full-sized cells connected in series, producing 6-12V and 10-60 watts.
In applications requiring more power than a single solar panel can provide, larger systems are made by linking together a number of solar panels. For example, a solar array consisting of 90 solar panels is sometimes used in building construction for the dual purposes of providing a wall and providing electric power to the building.
FIG. 1 shows the arrangement of a typical conventional solar cell array 100. As can be seen, the array 100 comprises M rows, each row having N solar cells 110 connected in series, wherein the M rows are all connected together in parallel. If all of the solar cells 110 have an exact same output voltage of P volts, and each solar cell 110 produces an exact same output current of R mA, then the output voltage of the solar cell array 100 would be N*P volts, and the output current would be M*R mA. So the output voltages and current capabilities of multiple solar cells 110 may be combined via the solar cell array 100.
However, there are certain shortcomings produced by the series/parallel configured solar cell array 100 shown in FIG. 1.
First, in the case of a failure of any single solar cell, an entire row of solar cells in the solar cell array 100 is lost. That is, the array has a single-point failure, which is undesirable.
Second, the output voltages of the solar cells are not all exactly the same. Typically, there is a voltage variation between solar cells, depending upon the variation of the solar cells. This voltage variation between solar cells causes a drop in total output power in the solar cell array 100 as the lower voltage solar cell rows are limited by the higher voltage solar cell rows (i.e., the xe2x80x9cbottleneckxe2x80x9d row(s)).
Third, whenever sunlight is not cast evenly upon the solar cell array 100, such as when part of the solar cell array 100 is in the shade or a shadow, the solar cells receiving more light will produce a greater current than the solar cells receiving less light. In that case, the current output by each row of the array will be limited to the lowest current produced by any one solar cell in the row (i.e., the xe2x80x9cbottleneckxe2x80x9d solar cell). This, in turn, causes a drop in output power for the solar cell array 100.
Also, whenever a certain solar panel in the solar cell array 100 is replaced due to failure or damage, the new solar panel is likely to have a different terminal voltage, affecting the output voltage and current of the solar cell array 100.
Accordingly, it would be desirable to provide a system and method for arranging solar cells in an array having a power output that is substantially less limited by bottleneck solar cells and rows or panels. The present invention is directed to addressing one or more of the preceding concerns by providing new connections.
In one aspect of the invention, a solar cell array, comprises a first four-port unit having first and second input ports and first and second output ports; and a second four-port unit also having first and second input ports and first and second output ports, wherein the first and second output ports of the first four-port unit are connected to the first and second input ports of the second four-port unit, and wherein each four-port unit comprises, a first solar cell device connected between the first input port and the first output port, a second solar cell device connected between the first input port and the second output port, a third solar cell device connected between the second input port and the first output port, and a fourth solar cell device connected between the second input port and the second output port.
In another aspect of the invention, a solar cell array comprises a plurality of electrically-conductive branches, said branches coupled in parallel, each of said branches comprising at least one solar cell device; and a plurality of shunts, wherein each one of said shunts couples an low voltage terminal of a solar cell device in one of said branches directly to a high voltage terminal of a corresponding solar cell device in an adjacent one of said branches, such that a corresponding set of solar cell devices together with their corresponding coupling shunts define a lattice-connected unit, and wherein said system comprises at least two said units, and said branches along with said shunts are coupled to form a cascaded-cell lattice arrangement having a respective node in each branch between adjoining cells.
In still another aspect of the invention, a method of producing electrical power, comprises the steps of coupling in parallel a plurality of electrically-conductive branches, with said branches forming at least two cascaded units having a respective node in each branch between adjoining units, wherein in each said unit, each said branch has a solar cell device producing electrical power and having a low voltage terminal and a high voltage terminal; and within each unit, coupling the low voltage terminal of each said solar cell device directly to the high voltage terminal of a solar cell device of an adjacent branch via a shunt; and providing the electrical power from the solar cell devices through input and output terminals connected to said branches.